Generally, in laser devices to obtain a laser output by gas discharge excitation, there is a relationship such that a larger amount of electric power input for the gas discharge provides a higher laser output and a smaller input provides a lower laser output. However, the relationship between the amount of power input and the laser output is not in direct proportion and is affected by the temperature of laser gas. This relationship is illustrated in FIG. 2 with a horizontal axis indicating an amount of power input for gas discharge in terms of discharge current (A: ampere) and the vertical axis indicating a laser output (W: watt).
As indicated by the solid line in the graph of FIG. 2, while the laser gas is cold, no laser oscillation takes place within a range H1 of discharge current from "0" to a threshold indicated by point P, and once the discharge current exceeds the threshold indicated by the point P, the laser output increases linearly with the discharge current. On the other hand, while the laser gas is hot, the threshold for laser oscillation drops to a level indicated by point Q, as shown by the broken line in the graph of FIG. 2. The expression "cold" used here means a state of "ordinary temperature or thereabouts" and is typified by a temperature range of 280 to 320 K (7 to 47.degree. C.). Also, the term "hot" means a state of "sufficiently higher temperature than ordinary temperature" and is typified by a temperature range of 400 to 500 K (127 to 227.degree. C.).
The characteristic in the vicinity of the point Q to P (range H2) is in actuality considerably unstable and it is not certain whether laser oscillation actually takes place at an arbitrary point (e.g., point S1) within the range H2. In general, in a case where a laser output command is switched from ON to OFF and thus the discharge current value drops from a high value (e.g., point S2) to a lower value point within the range H2), the laser oscillation shows a tendency to continue, as described later.
As the discharge current becomes much larger than the threshold indicated by the point Q, the discharge current/laser output characteristic gradually becomes closer to that (solid line) as observed while the laser gas is cold. Symbol W1 in the graph represents a laser output while the laser gas is hot in supplying a discharge current equivalent to the threshold while the laser gas is cold, and W1&gt;0.
In actually using a gas discharge laser, such control is performed as to switch between an OFF state in which a small discharge current called base current is supplied and an ON state in which a current to obtain a laser output required for machining or the like is supplied. In pulse operation mode, the two states are periodically and repeatedly alternated.
To smoothly carry out such control operation, it is necessary to set the base current to an appropriate value. If the base current value is set to a small value close to "0" as indicated by symbol R in FIG. 2, it is expedient for the laser output to be surely switched to the OFF state, but lowering of the gas temperature is unavoidable, making it difficult to maintain the gas discharge state. Once the gas discharge state is lost, special control is required again to restore gas discharge.
Specifically, it is necessary to apply a voltage much higher than that required to maintain the gas discharge state for inducing the gas discharge, and to control transition to the gas discharge-maintained state. It is apparently disadvantageous to perform such complicated and time-consuming control operation each time the laser is switched on and off.
Usually, therefore, the base current value is set to a value close to the point P (represented by the point S1). While a command to turn the laser output to the OFF state is output from a control section after the start of discharge, the gas discharge is maintained by supplying the base current set at the point S1, and when a command to turn the laser output to the ON state is output, a discharge current (represented by S2) large enough to obtain a required laser output W2 is supplied. In the pulse operation mode, the discharge current is controlled to be periodically altered between the points S1 and S2.
If the point S1 indicative of the base current is set at a value close to the point P, there arises a problem that the point S1 falls within the range H2 between the points Q and P. While the temperature of the laser gas is low, ON/OFF of an actual laser output (hereinafter referred to as "actual output") accurately responds to ON/OFF of the output command because the point S1 is closer to the origin than the point P. However, as the temperature of the laser gas rises due to a long-term operation or a high-output operation, the point Q indicative of the threshold for the actual output is shifted to the point S1 or to a position closer to the origin than the point S1, thereby accurate response of ON/OFF of the actual output to ON/OFF of the output command is not maintained.
In the range H2 from the point Q to the point P, the characteristic is considerably unstable as mentioned before, and the practical problem is that the actual output does not immediately become "0" when the laser output command is turned from ON to OFF.
Two charts in FIG. 3 illustrate a general relationship between the output command and the actual output observed in a situation where the above phenomenon has occurred. The horizontal axes in these upper and lower charts indicate the common time t (sec), the vertical axis in the upper chart indicates a voltage (Voc (mV) of the output command) for controlling the discharge current, and the vertical axis in the lower chart indicates the laser output W (watt).
As shown in the upper chart, the laser output control following the start of discharge is performed by switching the output command voltage Voc between two levels V1 and V2 as time elapses. For the pulse operation, the switching between V1 and V2 is repeated at shorter intervals. V1 represents a voltage of the output command for OFF, and is set to a value slightly smaller than a standard threshold Vth for laser oscillation (which corresponds to the point P in the graph of FIG. 2). A voltage V2 of the output command for ON is set to a value for obtaining the required laser output W2 (which corresponds to the point S2 in the graph of FIG. 2).
When the output command voltage Voc is applied as shown in the upper chart, the laser output generally changes as indicated in the lower chart. Specifically, when the output command voltage is turned from V1 to V2 at time ta, the laser output immediately rises. While the output command voltage is kept at V2 (e.g. at time tb), the laser output is substantially maintained at the fixed value W2.
However, when the output command voltage is then altered from V2 to V1 at time tc, the laser output does not immediately cease but trails to time td which can not be regarded as equivalent to tc, as indicated by symbol G. Thus, the laser oscillator with the conventional method of control is inadequate in actual output response to the laser oscillation stopping command, to cause a machining deficiency in laser beam machining, for instance.